1. Field of the Invention
The present invention relates to a spindle motor, which is suitably used in an apparatus installed with a hard disc or a laser beam printer.
2. Description of the Related Art
FIG. 6 is a cross sectional view of a conventional spindle motor according to the prior art. The conventional spindle motor has been used in an apparatus installed with a hard disc (hereinafter referred to as hard disc apparatus).
In FIG. 6, the conventional spindle motor is composed of a rotor 150R and a stator 150S. The rotor 150R is further composed of a shaft 101, a thrust plate 102 in disciform that is fixed to an end portion of the shaft 101, a hub 103 that is fixed to another end portion of the shaft 101, and a magnet 104 in a cylindrical shape that is magnetized in multi-magnetic poles and fixed to an inner circumferential surface of the hub 103.
On the other hand, the stator 150S is composed of a sleeve 105 in a cylindrical shape into which the shaft 101 is inserted so as to be rotatable freely, a thrust cover 106 in a flat plate that is fixed to the sleeve 105 with sandwiching the thrust plate 102 between the thrust cover 106 and the sleeve 105, a motor base 107 that sustains the sleeve 105, a core 108 having a radial protrusion that is formed with laminating a plurality of layers of a silicon steel plate and fixed to the motor base 107, and a coil 109 that is wound around the radial protrusion of the core 108.
A dynamic pressure groove is formed on either an outer circumferential surface of the shaft 101 or an inner circumferential surface of the sleeve 105, and a radial dynamic pressure bearing is constituted between the outer circumferential surface of the shaft 101 and the inner circumferential surface of the sleeve 105.
On the contrary, a thrust dynamic pressure bearing is constituted such that a groove, which generates dynamic pressure, is respectively formed on at least either one surface of the thrust plate 102 and the sleeve 105, which confront with each other, and at least either one surface of the thrust plate 102 and the thrust cover 106, which confront with each other.
Further, lubricant is filled into a void surrounded by the shaft 101, the thrust plate 102, the sleeve 105 and the thrust cover 106, and resulting in constituting a thrust dynamic pressure bearing.
Furthermore, an inner circumferential surface of the sleeve 105 is provided with a tapered section 110 at a top end portion of the sleeve 105 confronting the hub 103, wherein an inner diameter of the tapered section 110 increases in accordance with a direction toward a top end surface of the sleeve 105. By controlling an amount of lubricant in order to adjust liquid level of the lubricant to reach as high as a middle of the tapered section 110, the lubricant is prevented from leaking out of the spindle motor by an action of surface tension. In this connection, the tapered section 110 is a so-called taper seal section.
In the meanwhile, it is necessary for a total thickness of the conventional spindle motor shown in FIG. 6 to exceed a certain value, which is a sum of a thickness of a dynamic pressure bearing and a thickness d2 of a flat panel portion of the hub 103. In other words, the total thickness of the conventional spindle motor shown in FIG. 6 is necessary to be more than a total dimension in an axial direction of the shaft 101 that is a sum of thicknesses of the thrust cover 106 and the thrust plate 102 and a distance d1 added with the thickness d2, wherein the distance d1 is a distance between a top edge of a upper dynamic pressure groove and a bottom edge of a lower dynamic pressure groove.
It has been required for a spindle motor installed in a hard disc apparatus to be thinner in profile. However, in case that each thickness of the thrust cover 106 and the thrust plate 102 is thinned, the thrust dynamic pressure bearing is degraded in stiffness and accuracy.
Further, in case that the distance d1 between the dynamic pressure grooves is narrowed, the radial dynamic pressure bearing is deteriorated in stiffness, and resulted in degrading accuracy of radial run-out of the rotor 150R.
Furthermore, in case that a length of the tapered section 110 of the sleeve 105 in a longitudinal direction along the shaft 101 is shortened, the lubricant is easily affected by change of thermal expansion caused by temperature change, and resulting in leaking out when the liquid level of the lubricant exceeds the taper seal section 110 due to thermal expansion caused by high temperature.
On the contrary, in case of low temperature, the lubricant shrinks and disables to penetrate sufficiently into the radial dynamic pressure bearing section, and resulting in deteriorating accuracy of radial run-out of the radial dynamic pressure bearing or shortening life of the radial dynamic pressure bearing due to excessive load on the radial dynamic pressure bearing by possible damage to the shaft 101 and the sleeve 105 caused by hard contacting between them.
More, in case that the thickness d2 of the hub 103 is thinned, there exists a further problem such that stiffness of the rotor 150R is deteriorated.
In a hard disc apparatus, recording density of a hard disc installed in the hard disc apparatus is prevented from being improved in case that accuracy of radial run-out of a spindle motor to be installed in the hard disc apparatus is deteriorated and stiffness of each section of the spindle motor is degraded.
In order to improve the above-mentioned problems, the Japanese publication of unexamined patent applications No. 2002-54636 disclosed the bearing device of which profile was intended to be thinner. The bearing device is provided with the taper seal section that is formed on both the end surface of the sleeve, which confronts the hub, and the outer circumferential surface of the sleeve, which directly contacts with the end surface.
Generally, lubricant filled in a dynamic pressure bearing gradually evaporates and decreases in amount with time.
In the case of the bearing device disclosed in the Japanese publication of unexamined patent applications No. 2002-54636, the distance from the radial dynamic pressure bearing section to the taper seal section is extremely short. Therefore, the lubricant filled in the groove for generating dynamic pressure, which is provided in the taper seal section side, easily runs short in a minute due to the evaporation of the lubricant, and resulting in a problem such that the life of the radial dynamic pressure bearing is relatively short.
Further, the taper seal section provided in the outer circumferential area of the sleeve opens toward the upper surface of the stepped section of the sleeve, which is disposed under the taper seal section, so that the liquid level of the lubricant is hard to be confirmed visually when lubricant is filled in the bearing device while manufacturing a motor installed with the bearing device. Accordingly, there existed another problem such that filling work of lubricant is extremely hard.